This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Neuropilin is an essential bi-functional mammalian cell surface receptor functioning in both angiogenesis and axon guidance. Neuropilin is of particular medical importance since in addition to its normal roles, it also is required for VEGF dependent tumor angiogenesis, facilitating neo-vascularization and growth of solid tumors. During both normal and tumor-angiogenesis, neuropilin binds members of the VEGF family of ligands and functions as a co-receptor for the VEGF-R receptor tyrosine kinases. During neural development, neuropilin binds members of the semaphorin III family of ligands and functions as a co-receptor for members of the plexin family of receptors. We seek to understand neuropilin ligand binding, specificity, and receptor activation. Additionally, we seek to inform ongoing studies to define novel anti-angiogenesis targets. Fundamentally, our goal will be to answer the following questions: How does neuropilin activate and regulate VEGF dependent angiogenesis and semaphorin dependent axon guidance? Structural studies of neuropilin ligand binding will allow definition of the physical basis for interaction between core neuropilin and ligand domains. These studies will define common features of ligand binding as well as unique features that determine specificity between different protein homologues and isoforms. Further studies will characterize the mechanism(s) by which ligand binding to neuropilin couples to activation of VEGF-R and plexin signaling receptors. Structural studies will primarily be based on X-ray crystallography, but will also employ a variety of other structural techniques including NMR, EM, and SAXS. The latter two techniques will be particularly employed to study large and conformationally labile protein complexes. In addition, other biophysical and biochemical techniques will be employed to characterize protein-protein interactions and the effects of mutations in this system.